While the device of this invention is applicable for the long-term delivery of various drugs to a wide variety of animals, not excluding human beings, it is believed particularly useful for treating domestic animals for the purpose of reversibly controlling reproduction. The need is particularly acute with regard to control of the canine population in view of the increasing number of unwanted stray and abandoned dogs in this country and elsewhere. Such free-roaming animals have been known to cause property damage and attack other animals (including man) as well as carrying and spreading diseases. Few methods are presently available for successfully controlling canine reproduction.
Ovariohysterectomy (spaying) constitutes a positive method of reproduction control; however, ovariohysterectomy has met with opposition because of its irreversibility (especially with respect to pets) and because of its cost (primarily with respect to stray or abandoned animals). As an alternative to spaying, mechanical devices have recently become available which are designed to prevent penile insertion into the female but, unfortunately, such devices are of limited effectiveness.
Chemosterilants have been studied and found to control fertility, but they often cause pathological changes in the uterus making ovariohysterectomy necessary. All known exogenous progestens cause cystic endometrial hyperplasia, endometritis, and mucometra. Of various progestens that have been studied, only magestrol acetate is approved for commercial use and, in addition to the disadvantages already mentioned, is relatively expensive.
The problems with chemosterilants is not only one of drug selection but also of ease and effectiveness of administration. Thus, subcutaneous androgen implants such as testosterone have been tested in the bitch and found to inhibit estrus with minimum side effects. Implantation is, however, a surgical procedure usually requiring the administration of anesthesia and, depending on the weight of the animal, more than one implant may be necessary. Oral administration, although simpler, is time-consuming and inconvenient since estrus is inhibited only for the duration of a daily dosage which must be initiated prior to the beginning of proestrus to be effective.
Osmotic devices for delivering drugs in various selected environments have been disclosed, for example, in U.S. Pat. Nos. 4,160,452, 4,036,227, and 4,014,334. While such devices are intended to deliver drugs at continuous and predetermined rates for extended periods, their effectiveness depends on osmotic pressure which in turn depends on factors which may be variable. The presence and extent of fluid surrounding such devices, and the precise nature of such fluid, all affect the operation of osmotic devices.
Other patents indicating the state of the art are U.S. Pat. Nos. 3,659,600, 3,984,263, 3,492,993, 1,476,946, and 3,929,132, and the references cited therein.
In view of the above, it is a main object of this invention to provide a long-term drug delivery device suitable for intrauterine or intravaginal (preferably the latter) placement, which is positive-acting in its operation. Specifically, it is an object to provide an intravaginal or intrauterine device which utilizes a positive pressure mechanism for the gradual and continuous delivery of a drug to a warm-blooded animal.
The positive pressure mechanism includes a compression spring which exerts a constant force to reduce the size of a variable-volume chamber containing a drug solution which is capable of passing through a diffusion membrane and of being absorbed by the vaginal or uterine epithelium. One wall of the chamber includes a supported diffusion membrane; a second wall of the chamber, or a compartment of that chamber, takes the form of a plunger driven by the compression spring. The casing defining the chamber is preferably of cylindrical shape and is dimensioned to be received within the body cavity of the animal to be treated. Where the cavity constitutes the vaginal cavity, the device is additionally provided with flexible retaining elements which project radially outwardly from the end of the casing opposite from the end in which the diffusion membrane is supported. The result is a dependable and relatively simple drug-delivery device which utilizes a positive-pressure mechanism, not dependent upon osmotic pressure, and which is particularly suitable for delivering estrus-controlling drugs to livestock, pets, and other warm-blooded animals.
Other features, objects, and advantages of the invention will become apparent from the drawings and specification.